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I ran into this a few months back. You can buy a certificate for whatever you want! However a few things to keep in mind...

You cannot buy a wildcart cert for your third level domain and have it protect your fourth level domains, you have to secure the domains at each level of the domain name tree. In general the SSL providers won't restrict how many levels you can have though.

We had a wildcard cert for our third level domain name, something like *.us.mydomain.com which allowed us to secure our US resources, but we also needed one for our Polish resources so another cert for *.pl.mydomain.com was required.

Some of the providers may limit to what depth you can go though, but the browsers should all support it.